1. The Field of the Invention
The present invention generally relates to x-ray tubes. More specifically, the present invention relates to an apparatus for reducing contaminating secondary x-ray emission from stationary anode x-ray tubes.
2. The Relevant Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. Such equipment is commonly used in applications such as diagnostic and therapeutic radiology, semiconductor fabrication, joint analysis, and non-destructive materials testing. While used in a number of different applications, the basic operation of an x-ray tube is similar. In general, x-rays are produced when electrons are accelerated and impinged upon a material of a particular composition.
X-ray generating devices typically include an electron source, or cathode, and an anode disposed within an evacuated enclosure. The anode includes a target surface that is oriented to receive electrons emitted by the cathode. In operation, an electric current is applied to a filament portion of the cathode, which causes electrons to be emitted by thermionic emission. The electrons are then accelerated towards the target surface of the anode by applying a high voltage potential between the cathode and the anode. Upon striking the anode target surface, some of the resulting kinetic energy is released as electromagnetic radiation of very high frequency, i.e., x-rays.
The specific frequency or wavelength of the x-rays produced depends in large part on the type of material used to form the anode target surface. Anode target surface materials with high atomic numbers (xe2x80x9cZxe2x80x9d numbers), such as tungsten, are typically employed. The x-rays ultimately exit the x-ray tube through a window in the x-ray tube, and interact in or on various material samples or patients. As is well known, the x-rays can be used for sample analysis procedures, therapeutic treatment, or in medical diagnostic applications.
One application for which x-ray tubes are well suited is referred to as x-ray fluorescence spectroscopy (xe2x80x9cXRFxe2x80x9d). XRF is typically used to determine the elemental composition of a selected material. An XRF instrument setup typically includes an analytical x-ray tube (AXT), a specimen to be analyzed, a collimator, a diffracting crystal, and an x-ray detector. To analyze the composition of the specimen, the x-ray tube is activated and x-rays are directed at the specimen. The interaction of the x-rays with the atoms in the specimen causes the atoms to emit, or fluoresce, a second group of excited x-rays having energies characteristic of the elements in the specimen. Once emitted by the sample, the fluoresced x-rays are dispersed into a spectrum by a diffracting crystal, and are then collimated towards a detector and associated instrumentation, which quantifies and correlates the results. The intensities of the various energy peaks in the spectrum are roughly proportional to the concentration of the corresponding elements that comprise the specimen. In this way, the elemental composition of a variety of materials may be ascertained.
Many x-ray tubes employ a rotary anode that rotates portions of its target surface into and out of the stream of electrons produced by the cathode. However, analytical x-ray tubes, such as those used for XRF applications, typically use a stationary anode. The stationary anode typically includes a substrate portion, comprised of copper or similar material, and a target surface comprised of rhodium, palladium, tungsten, or other suitable material. For an XRF procedure to yield superior results when assaying a specimen, it is highly desirable that the x-ray tube produce a stream of primary x-rays that is spectrally pure, i.e., the spectrum is comprised of the continuous spectrum and the characteristic peaks of the target material. This spectrally pure stream of primary x-rays is produced by those electrons that impact the target surface of the anode and produce x-rays having a characteristic wavelength corresponding to the material deposited on the target surface.
Unfortunately, many of the electrons that impact that target surface do not produce primary x-rays. Most of the kinetic energy that results from the impact is released in the form of heat. Also, a significant number of electrons simply rebound from the anode target surface and strike other non-target surfaces within the x-ray tube, such as the anode substrate or other components within the tube. These electrons are often referred to as xe2x80x9cback-scatterxe2x80x9d or secondary electrons. These back-scattered electrons retain a significant amount of their original kinetic energy after rebounding. As such, these secondary collisions with non-target surfaces can produce secondary or off-focus x-rays having a wavelength that is characteristic of the material impinged, such as copper. These secondary x-rays are emitted from the x-ray tube along with the primary x-rays created at the target surface of the stationary anode. In XRF spectroscopy, these secondary x-rays are considered an undesirable contamination of the primary x-ray stream because they interfere with the measurement of the fluorescing x-rays emanating from the specimen under analysis. In other words, an XRF detector may mistake a contaminating secondary x-ray having, for example, a characteristic copper wavelength produced by the copper anode substrate as having been produced by a fluorescing copper atom present in the specimen under analysis. Thus, to optimize the quality of the signal, it is critical to reduce or eliminate these secondary x-rays from the x-ray emissions of an x-ray tube.
Several attempts have been made to eliminate secondary x-ray contamination from primary x-ray emissions. One approach involves the process of chemically coating the anode substrate with the same material as is deposited on the target surface. This method has met with only limited success due to the difficulty in getting a sufficiently thick plating to adhere to the anode substrate. Moreover, during tube operation, the high temperatures present in the anode substrate often cause the coating to intermingle with the substrate material, leading to the eventual production of contaminating secondary x-rays.
Another approach has involved the use of a graphite layer to cover a portion of the anode substrate where back-scattered electrons typically impact. Though this approach reduces the amount of contaminating x-rays that are emitted, it gives rise to other problems. In particular, the approach results in serious outgassing and particle creation problems during tube operation because of differing thermal expansion rates between the graphite layer and the anode substrate, and because of the extensive machining and handling steps required for assembly and attachment of the graphite layer. Outgassing and particle creation within the evacuated environment of an x-ray tube are highly detrimental to its performance and operating lifetime
A need therefore exists for a stationary x-ray tube that reduces or eliminates the production of secondary x-rays. This need is especially acute in x-ray tubes employed in XRF spectroscopy operations, which require spectrally pure x-ray signals. Further, any solution to enable the creation of spectrally pure x-ray streams should do so without creating ensuing problems, such as outgassing and particle creation that are detrimental to the operation of the tube.
Embodiments of the present invention have been developed in response to the current state of the art, and in particular, in response to these and other problems and needs that have not been fully or adequately solved by currently available x-ray tubes. In general, embodiments of the present invention are directed to a cover, or sleeve, that reduces or eliminates the emission of secondary x-ray contamination in stationary anode x-ray tubes. In addition, the sleeve is implemented in a manner so as to prevent other problems within the tube, such as contamination and outgassing. In preferred embodiments, the sleeve is sized and configured to cover a portion of a component disposed within the x-ray tube evacuated enclosure that is susceptible to being impinged by secondary or back-scattered electrons. For example, in one embodiment, the sleeve is affixed to a portion of the stationary anode substrate that is adjacent to the target surface. In particular, the sleeve is positioned so that it prevents errant electrons back-scattered from the target surface from impacting the anode substrate, thereby preventing the production of secondary x-rays. Instead, the back-scattered electrons that would otherwise impact the anode substrate impact the anode sleeve, and produce x-rays that are within a wavelength range that do not negatively impact the analysis being undertaken.
Preferred embodiments of the anode sleeve generally comprises a shape necessary to cover a portion of the outer surface of the anode substrate. For example, in one embodiment, the anode sleeve is formed in the shape of a hollow cylindrical body. The anode sleeve has a cylindrical length sufficient to cover those portions of the anode substrate that are susceptible to impact by back-scattered electrons. For example, in some applications, the anode sleeve covers a small portion of the anode substrate adjacent to the target surface. Alternatively, if the application dictates, the length of the sleeve may be greater so as to cover a greater portion of the anode substrate. The thickness of the anode sleeve wall(s) need only be thick enough to prevent penetration of electrons to the anode substrate material. The sleeve is preferably comprised of a material that does not create contaminating x-rays as detected by analysis instrumentation when impacted by electrons. For example, in a preferred embodiment, the anode sleeve comprises beryllium. Other suitable materials could be used depending on the functional requirements of the x-ray tube and the analysis being performed.
Embodiments of the present invention use the anode sleeve on a stationary anode in an x-ray tube having an end-window configuration. Alternative embodiments use the sleeve in x-ray tubes having a side-window configuration. Indeed, the anode sleeve of the present invention may be adapted in size and shape to fit a variety of anodes and types of x-ray tubes. Also, a sleeve or a cover could be fitted to other interior x-ray tube components to prevent secondary x-ray emissions from those components as well. An example of this would include a cathode shield comprising beryllium that is positioned so as to prevent secondary x-rays from being produced from portions of the cathode.
The present anode sleeve makes possible the production of spectrally pure primary x-ray streams by reducing or eliminating the production of secondary x-ray signals. Inaccuracies created by such contamination in sensitive analysis procedures, such as XRF spectroscopy, are significantly reduced or eliminated. Therefore, the composition of samples subjected to XRF spectroscopy may be determined with greater precision that what was before possible. Additionally, forming the sleeve from beryllium or similar materials avoids the problems associated with outgassing and particle creation encountered with prior art solutions.
These and other objects, features and advantages of the present invention will more fully apparent from the following description and appended claims, or may be by the practice of the invention as set forth hereinafter.